The separation of trivalent actinides like americium or curium, from trivalent lanthanides has long been a problem. The reason for the difficulties in the separation of these elements is the very close chemical properties of the trivalent ions of the lanthanides and actinides. Especially the very similar ionic radii contribute to similar properties of these element groups. Thus numerous investigations have hitherto been undertaken to separate the trivalent ions of the actinides and lanthanides with the highest possible selectivity and increased efficiency.
It is indeed known that extraction agents with mild donor groups like nitrogen or sulphur as complex formers contain structural components which in liquid-liquid extraction have a certain selectivity with respect to trivalent actinides although up to now such separations have been carried out only at relatively high pH values which tend to precipitate formation of the trivalent actinides or require 10 molar LiCl solutions.
In the publication K. L. Nash, Solvent Extr. Ion Exch. 11 (4), 729-768 (1993) the Talspeak process is described and enables selective extraction of lanthanides with the aid of complexing agents which maintain the trivalent actinides in solution. However, this separating process is also carried out at a relatively high pH value of 3-4 and requires the addition of further salts.
A process developed by the Applicant for the extraction of trivalent actinides from aqueous solutions which contain trivalent actinides and trivalent lanthanides, enables a separation at high acid concentrations of 0.01 to 2 mol/liter HNO.sub.3. Following this process, such an aqueous acid solution which contains a mixture of trivalent lanthanides and actinides is extracted by means of an organic solvent. With this process, because of the low pH value or the high acid concentration, a precipitate formation of the trivalent actinides is hindered and good separation results are obtainable. To evaluate the separation results, characteristic values such as the distribution coefficient D and the separation factor .alpha. are considered EQU D.sub.An(III) =[An(III).sub.org ]/[An(III).sub.w ] (1)
In formula 1:
D.sub.An(III) =distribution coefficient for a trivalent actinide (dimensionless) PA1 [An(III).sub.org ]=concentration of the trivalent actinide in the organic phase (mole/liter) PA1 [An(III).sub.w ]=concentration of the trivalent actinide in the aqueous phase EQU D.sub.Ln((III) =[Ln(III).sub.org ]/[Ln(III).sub.w ] (2) PA1 D.sub.Ln(III) =Distribution coefficient for a trivalent lanthanide (dimensionless) PA1 [Ln(III).sub.org ]=concentration of the trivalent lanthanide in the organic phase (mole/liter) PA1 [Ln(III).sub.w ]=concentration of the trivalent lanthanide in the aqueous phase. EQU .alpha.=D.sub.An(III) /D.sub.Ln(III) PA1 R.sub.1 =phenyl or naphthyl, PA1 R.sub.2 =phenyl or naphthyl, as well as methyl-, ethyl-, propyl-, isopropyl-, cyano-, nitro-, halogenyl- (Cl--, F--, Br--, J--) substituted residues of R.sub.1 and R.sub.2 whereby R.sub.1 and R.sub.2 can be substituted with at least one of the components from the group of methyl, ethyl, propyl, isopropyl, cyano, nitro, halogenyl (Cl, F, Br, I).
In formula 2:
.alpha.=separating factor (dimensionless) depending upon the process, the use of bis(aryl)dithiophosphonic acid in strongly acid medium can produce separating factors .alpha. lying between about 20-50.